Traction control systems are commonly integrated into anti-lock braking systems. Anti-lock braking systems typically modulate the pressure of hydraulic fluid delivered to a vehicle wheel brake to prevent the vehicle wheel from locking up in the braking condition. Conversely, a traction control system, when integrated into the anti-lock braking system, actuates the brakes to prevent spinning of a vehicle wheel, thereby maximizing the traction which can be exerted by that wheel.
On-demand four wheel drive systems enable an automatic switching from two wheel drive used during normal operating conditions to four wheel drive responsive to slippage of the primary axle used for two wheel drive. On-demand four wheel drive systems can also be placed in the four wheel drive mode by the driver actively selecting that mode with a switch or button.
Known traction control systems for four wheel drive vehicles operate independently of engagement of the secondary axle through a transfer case. The benefit of traction control in preventing slipping, and maintaining the stability of vehicle handling under slippery conditions, for a four wheel drive vehicle is greatly reduced when the four wheel drive system is operating in a two wheel drive mode, particularly if the rear wheels are being driven in the two wheel drive mode.
Four sensor anti-lock systems employ a wheel rotation speed sensor at each of the four vehicle wheels, enabling individual braking control of each wheel. Three sensor anti-lock braking systems are a less expensive alternative to the four sensor systems. With three sensor systems, each of the two front wheels in the vehicle has a wheel speed sensor, and a third sensor is used to monitor the speed of the rear drive shaft. Given that the rolling radius of the vehicle wheels and the final drive axle ratio for the front and rear axles are known, the speeds of the wheels and the axle shaft can be compared to determine if there is any incipient wheel lock up occurring in either of the front wheels, or of either of the rear wheels during a brake application. If such incipient lock up is detected, the front two wheels can be controlled individually, and the two rear wheels can be controlled simultaneously with each other by selectively relieving brake pressure.
It is desired to provide a traction control system compatible with on-demand four wheel drive systems and further compatible with both three sensor and four sensor anti-lock brake systems.